The best way to bend the bedwalls inward is with a large ratchet strap hooked into the body crossmember at the bottom rear of each quarter panel. Use the ratchet only to take the slack out, then sit on the strap to bend the body. If only one side needs to be bent, install a brace (a 2x4 cut to fit will work) from the bottom of that side to the top of the side that doesn't need adjustment.
Check progress frequently.
http://www.fourdoorbronco.com/board/showthread.php?t=5224 
